Thermal safety plug



pri 17, 1956 c. B. LIVERS 2,742,179

THERMAL SAFETY PLUG Filed Jan. 21, 1955 /5- 501. DE 1? log new Ill/ll:SOLDER INVENTOR. C. B. L/VERS ATTORNEY r 2,142,112 r SAFETY PLUG:

Carlos B. Livers, North Hollywood, Calif., assignor to Bendix AviationCorporation, North. Hollywood, Calif., a corporation of Delaware 1 vApplication January 21,193,. Serial No. 483,301 1 Claim. (craze-:39

This invention relates" to safety plugs that blowout in response to arise in tempe'ra-ture above a predetermined value to relievepressure inasystem and prevent damage thereto. The invention is particularly usefulin, although not limited to, hydraulic power systems to unl'oad'the pumpifthe temperature of the fiuidlrisesto a value such that continuedoperation under pressure might damage the pump.

An object of theinvent-ion is to provide a simple and reliable thermalsafety plug that is relatively insensitive to pressure while at the sametime providing a large passage for escape of fluid when it opens.

Other more specific objects and features of the invention will appearfrom the description to follow.

Patented Apr. 17, 1956 screwconnection tothedevicetobe protected- Themain portionof thev bodymemberlll. may be. of .hexagonal external shape,as shown in Fig. l, for application of a wrench. The. constructiondisclosed provides both fluid connection between. the chamber. 11 andthe pressurefluid circuitof the. device to, bepmtected, and thermalconnection to. insure. that the body member 10 will follow thetemperature of the device- The-body 10. isprovidedwith two coaxialventholes or. passages, 12 and 13, respectively, which. are of differentdiameters. and. extend: in opposite directions from the chamber. 11 tothe. exterior of the body. These vent holes 12 and 13 are normallyclosed by a single plug 14 having a: largev cylindrical end 14a. looselyfitting the passage. 1'2 anda. small cylindricalend 14b looselyv fittingthe passage 13. .Eachof theendportions 14a and 14b is secured in sealingrelation with,.its associated passage 12 andi13 by a film of solder15.fillingfthe. clearance space.

Pressure fluid inthe chamber, 11 exerts a pressure force on theplug14actingin upwardv direction (with reference to. Fig. 2. 01 the drawing)and. with force that is. proportionalto-the. pressure .andthedifierentialarea of the two It is old to provide in pressure systems avent hole nor- 7 mally closed by a loosely fitting plug soldered inplace with a low-melting solder that melts in response to a dangerousrise in temperature to permit release of the plug in response topressure thereon and vent fluid to reduce the pressure. In manyinstances, the vent hole must be quite large to handle the full fluidflow of the system and reduce the pressure to a safe value. The pressureforce acting on the plug is proportionate to its area, and in a highpressure system this pressure force on a large plug may be so great thatthe solder fails below the desired temperature because of deteriorationof the strength in response to aging and/ or softening at a temperaturebelow thefusing temperature.

This problem is solved in accordance with the present invention byproviding two diametrically opposite vent holes of diiferent diametersclosed by a single plug having end portions of diiferent diametersslidablyfitting in the respective vent holes and soldered in place witha solder having the desired fusing temperature. With this construction,the total resultant pressure force tending to eject the plug isproportional to the differential area of the two end portions.Therefore, the vent holes can be made as large as necessary to handlethe flow, while at the same time the blowout force can be made as smallas desired by choosing the relative diameters of the two holes such asto yield a desired differential area.

A full understanding of the invention may be had from the followingdetailed description with reference to the drawing, in which:

Fig. 1 is an end view of one embodiment of the invention.

Fig. 2 is a longitudinal section taken in the plane II-II of Fig. 1.

, circuit of a pressure fluid device to be protected. Thus,

the chamber 11 extends through one end 10a of the body member, and theadjacent end portion 10b is threaded for end'portions, 14a. and lfib, ofthe. plug. This. difierential area can be made as srriall as desired byvarying the relative diameters of the end portions 14a and 14b of theplug and their associated passages 12 and 13. In other words, thepressure force tending to eject the plug is not determined by theabsolute area of the vent passages 12 and 13, and they can be made aslarge as desired to provide the desired free flow of fluid therethrough.

The diiferential area between the plug ends 14a and 14b is so chosen,relative to the pressure that is normally existent in the chamber 11, asto produce a resultant pressure force on the plug 14 that is ample toeject the plug when the solder 15 has melted in response to a dangerousrise in temperature, but insufficient to eject the plug prior to meltingof the solder.

In practice, in a fitting as shown in Figs 1 and 2 in which the threadedportion 10b is of /s" pipe size, the large end portion 14a may have adiameter lying within the limits .131 and .132", and the diameter of thepassage 12 may lie within the limits .133" and .134", thereby providinga minimum clearance of .001" and a maximum clearance of .003 to befilled with the solder 15. If this fitting is designed for a workingpressure of 2000 p. s. i.,

the diameter of the small end portion 14b of the plug may able to insureejection of the plug in response to the.

pressure force resulting from the diiferential area between the two endportions. On the other hand, it is undesirable to have an excessiveclearance to be filled by a large quantity of solder.

In manufacture, the surfaces of the passages 12 and 13 in the bodymember and the surfaces of the plug 14 are tinned with the solder priorto assembly. Then the plug is inserted in the body member and theassembly heated above the melting point of the solder to cause it tomelt and flow between the plug and the passage surfaces by capillaryattraction. While the assembly is heated, any necessary additionalsolder can be added at the outer ends of the plug to completely fill theclearance.

Solders of diflerent compositions, having ditferent fusing points, arewell known, and a suitable composition is chosen according to themaximum temperature at which it is desired to have the plug blow out. Insome hydraulic applications, it is desirable to have the plug respond toany temperature in excess of about 280 F., and a suitable soldercomposition for this temperature consists of:

7 Percent, Tin 45 Lead 32 Cadmium '18 Bismuth In the form ofconstruction shown in Figs. 1' and 2, in which the chamber 11 is closedat one end, there would be little circulation of fluid between thechamber and the device to which it is connected, and thermal conductionis chiefly through the threaded connection between the safety'plug andthe device on which it is mounted. In

some instances, the body may be located within a.

' ends for connection into a pipe line, so that 'the fluid flows throughthe passage 21, and the temperature of the body 20 is determined by thetransfer of heat thereto from the fluid. The plug 14 can be extendedthrough the passage 21 if the fluid velocity is low, but it ispreferable to extend it through a lateral recess '22 which is incommunication with the passage 21, thereby leaving the latterunobstructed. A high velocity flow past the plug 14 could apply a sidethrust to it that would prevent its ejection when the solder melted.

Although for the purpose of explaining the invention a particularembodiment thereof has been shown and described, obvious modificationswill occur'to a person skilled in the art, and I do not desire to belimited to the exact details shown and described.

I claim: g

A thermal safety plug comprising:' a body member defining a chamberadapted to be connected to a pressure fluid device to be protected andhaving a pair of coaxial cylindrical vent passages of difierentdiameters extending in opposite directions from" said chamber throughsaid body member; a plug member extending through said chamber andhaving opposite cylindrical end portions extending into said respectivevent passages, each said end portion being of slightly smaller diameterthan its associated vent passage, whereby it has a free sliding fittherein; and an annular body ofsolder of predetermined melting pointlower than those of said body and plug members filling the clearancespace between each end portion of said plug member and its associatedbody passage.

No references citedk

